The present invention relates to novel copolymers of tetrafluoroethylene and ethylene and a process for producing the same.
More particularly, it relates to novel copolymers of tetrafluoroethylene and ethylene having improved properties which are produced by copolymerizing tetrafluoroethylene and ethylene with a small amount of a perfluoroalkyl vinyl monomer, and the process for producing the same. It has been known to produce copolymers of tetrafluoroethylene and ethylene which are characterized by excellent chemical resistance and excellent thermal stability and good insulating properties by copolymerizing tetrafluoroethylene and ethylene in the presence of a polymerizaion initiator. These copolymers are known to be useful as raw materials for preparing various shaped articles, electric wire coating and corrosion resistant linings, etc., because of their excellent properties, and significant melt processabiltiy.
It has been proposed to copolymerize tetrafluoroethylene and ethylene with an auxiliary amount of a vinyl comonomer which has no telogen activity and causes a side chain having at least two carbon atoms in order to improve physical characteristics at high temperature especially tensile characteristics at high temperature for copolymers of tetrafluoroethlene and ethylene (such as Japanese Patent Publication No. 23671/1972 and U.S. Pat. No. 3,624,250).
In accordance with these processes, it is possible to overcome the disadvantage that a coating of a copolymer of tetrafluoroethylene and ethylene on a wire becomes brittle at high temperature to cause cracks even under low stress.
However, when the vinyl comonomer which forms a side chain having at least two carbon atoms, is used, the advantageous effect can be attained for improvement of tensile characteristics at high temperatures but various disadvantages are found in industrial purposes.
For example, when the copolymerization of tetrafluoroethylene and ethylene is carried out by adding a perfluoroolefin having the formula EQU CF.sub.2 = CF -- C.sub.2 F.sub.5 and CF.sub.2 = CF -- C.sub.4 F.sub.9,
the rate (velocity) of the copolymerization is remarkably low.
When the polymerization of tetrafluoroethylene and ethylene is carried out by adding a vinyl ether having the formula EQU CF.sub.2 = CF -- OR.sub.f such as CF.sub.2 = CF -- OC.sub.3 F.sub.7 or EQU Cf.sub.2 = CF -- OCH.sub.2 R.sub.f or CF.sub.2 = CF -- OR,
the tensile creep property of the resulting copolymer is deteriorated. For example, the copolymers of tetrafluoroethylene and ethylene which have about 2 to 3% of initial strain in the tensile creep test at 175.degree. C under 30 Kg/cm.sup.2 are changed to those having higher than 7% of the initial strain by the introduction of the vinyl ether monomer. When a hydrofluoroolefin or a vinyl ester comonomer such as EQU CF.sub.2 = CF--CH.sub.2 --C(CF.sub.3).sub.2 OH or ##STR1## is a copolymerized, the resulting copolymer has a lower heat resistance, for example, the heat resistance of longer than 200 hours in an ageing test at 230.degree. C for measuring the period for which the elongation at 23.degree. becomes half compared with initial elongation is decreased to 80 to 100 hours or lower by the introduction of the side chain in the copolymer of tetrafluoroethylene and ethylene.